Method and apparatus for storing information on a storage medium

ABSTRACT

Apparatus and process for writing information using a laser light writing beam focused on a light sensitive layer having a threshold power level above which the layer reacts to the laser write beam. The intensity of the writing beam is adjusted relative to the threshold power level. In one embodiment, the peak intensity of the writing beam is adjusted so that one-half of the peak intensity is equal to the threshold power level. In a second embodiment, the average intensity of the writing beam is adjusted to equal the threshold power level.

This is a division of application Ser. No. 106,107, filed Dec. 21, 1979,which is a division of application Ser. No. 890,407, filed Mar. 27, 1978and now U.S. Pat. No. 4,225,873, which is a continuation-in-part ofapplication Ser. No. 714,133, filed Aug. 13, 1976, now abandoned, whichis a continuation of application Ser. No. 508,815, filed Sept. 24, 1974and now abandoned, which is a continuation of application Ser. No.333,560, filed Feb. 20, 1973 and now abandoned.

RELATED PATENT APPLICATIONS

Reference is to be had to the patent application entitled MASTERINGMACHINE by Richard L. Wilkinson, Ser. No. 890,771 filed Mar. 26, 1978,abandoned in favor of continuation application Ser. No. 94,108 filedNov. 14, 1979.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the writing of a frequency modulatedelectrical signal upon an information bearing video disc surface in theform of successive light reflective and light non-reflective regions.

A reading apparatus is described using a reading light beam movingrelative to the successive light reflective and light non-reflectiveregions to reconstruct the stored frequency modulated signal. Thereconstructed frequency modulated signal is demodulated in adiscriminator and changed to a form suitable for display on a T.V.monitor.

A novel information storage member is described for use in the writingand reading apparatus. This member includes successive light reflectiveand light non-reflective regions representing the frequency modulatedsignal.

Bias control circuitry for achieving duty cycle control in the writingapparatus and an improved form of a Pockels cell driving signal is alsodescribed.

2. Description of the Prior Art

U.S. Pat. No. 3,474,457 to C. H. Becker discloses an optical recordingand reproducing apparatus using a highly focused coherent laser lightbeam to selectively burn away discrete, uniform portions of a filmdeposited on a carrier member. Information to be recorded is applied toa high frequency oscillator having a high frequency pulse repetitionrate which is used to form holes in the carrier member. These holes areof substantially uniform and controlled dimensions of extremelydiminutive sizes.

The present invention differs significantly from this technique. One ofthese differences includes changing the information signal to berecorded into a frequency modulated signal having a carrier frequencyand frequency changes in time varying from the carrier frequency, andrecording this frequency modulated signal upon an information carrier inthe form of successive light reflective and light non-reflective regionsof variable length to represent the instantaneous frequency of thefrequency modulated signal.

U.S. Pat. No. 3,564,131 to E. D. Herold et al, and U.S. Pat. No.3,720,784 to D. Mayden et al, disclose a system for producing an actualimage comprising a multitude of small discrete holes formed by a laserin a radiation absorbing medium. These teach varying the spacing betweenthe discrete holes and varying the size of the holes, respectively, torepresent halftones in storing the actual image.

The present invention does not store an actual image. A typical signalsource for use with the present invention is a television camera or avideo tape recorder furnishing a recorded signal from a televisioncamera. This signal is commonly called a video signal and it appears asa one volt peak to peak electrical signal in the form of a voltagevarying with time format. This video signal is changed to a carrierfrequency having frequency changes in time varying about the carrierfrequency. This frequency modulated signal is stored on the informationbearing surface of a video disc in the form of successive lightreflective and light non-reflective regions of varying lengths torepresent the instantaneous frequency of the frequency modulated signal.

An improved embodiment of Applicant's writing and reading apparatus anddisc member are described in a publication entitled "A Review of the MCADisco-Vision System" published in the July, 1974 Journal of the SMPTE,Volume 83.

SUMMARY OF THE INVENTION

The present invention relates to a video disc writing and readingapparatus and method, and further relates to a video signal storagemember for use in the writing and reading apparatus.

The combination of the present invention includes a spindle whichrotates the disc precisely in a circle and a lead screw mechanism fortranslating the head at a very constant velocity along a radius of therotating disc. Obviously, it is desirable to synchronize or otherwisecoordinate the disc drive with the translating drive to create a spiraltrack of predetermined pitch. In a preferred embodiment, the spacingbetween adjacent turns of the spiral is 2 μm, center to center. Assuminga spot diameter of the 1 μm, there will be a guard area of 1 μm betweenspots in adjacent tracks.

The "writing head" in the preferred embodiment is a microscope objectivelens which flies at a constant height above the disc on an air bearing.The constant height is necessary because of the shallow focal depth ofthe objective lens A 40X dry microscope objective lens has been found tobe satisfactory in terms of concentrating the energy of the laser beamat the disc surface to enable the writing of the 1 μm spot. A portion ofthe writing beam is sensed by a novel Pockels cell stabilizing circuitwhich tends to maintain the average power of the modulated beam at apredetermined level.

An Argon ion laser is used as the writing beam. A Pockels cell and Glanprism combination modulates the laser beam with the video information.In accordance with the present invention, a "read after write"capability is provided to monitor the writing operation. A second,Helium-Neon (He.Ne) or "reading" laser is provided which directs a lowerpower beam into the writing beam path, but at a slight angle withrespect to the writing beam as it enters the writing head. The angle ischosen so that the reading beam illuminates an area on the track beingwritten that is approximately 2 μm downstream from the writing spot.

The reading beam is reflected from the disc surface and returns throughthe writing head, retracing its optical path to a dichroic mirror whichinitially inserted the reading beam into the writing path. The readingbeam is then directed to a beam splitter and through a bandpass filterwhich blocks any of the writing beam that may have followed the samepath. The reading beam then impinges on a photodetector, which togetherwith an FM discriminator generates the video information signal. Thequality of this signal, when displayed on an oscilloscope or a TVmonitor, indicates whether the values of the peak "cutting" power,average cutting power and focus are correct.

The "read after write" information can also be utilized in an errorchecking mode, especially if digital type information is being written.The input video information is delayed for an interval equivalent to thetime displacement between the writing spot and the reading spot. Thereturned information is then compared with the delayed input informationfor "identity." The existence of too many dissimilarities would be abasis for either rechecking and realigning the apparatus or rejectingthe disc.

The combination of the present invention includes a precision lathewhich rotates the disc in a perfect circle and translates the recording"head" at constant velocity along a radius of a rotating disc.Obviously, it is desirable to synchronize or otherwise coordinate thetwo drives so that a spiral track of predetermined pitch can be created.If desired, concentric circles can also be created by alternatelytranslating and writing. In a preferred embodiment employing a spiral,the spacing between adjacent turns of the spiral is 2 μm, center tocenter. Assuming a spot diameter of 1 μm, there will be a guard area of1 μm between spots in adjacent tracks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized block diagram of a mastering machine inaccordance with the present invention, for optically recording afrequency-modulated information signal on a storage member;

FIG. 2 is a schematic view of the optical path through the objectivelens of FIG. 1;

FIG. 3 is a schematic representation of the relative spacing between thepoints of impingement of the writing and reading beams;

FIG. 4 is a schematic diagram of a Pockels cell stabilizing circuit usedin the mastering machine of FIG. 1;

FIG. 5 shows various waveforms present in the mastering machine of FIG.1;

FIG. 6 is a schematic cross-sectional view of a metallic film-coatedvideo disc used with the mastering machine of FIG. 1;

FIG. 7 is a schematic, cross-sectional view of a photoresist-coatedvideo disc used with the mastering machine of FIG. 1;

FIG. 8 is a schematic, cross-sectional view of the photoresist-coatedvideo disc of FIG. 7, after exposed portions of it have been removed;

FIG. 9 is a graph of the transfer characteristic of the Pockels cell andGlan prism combination of the mastering machine of FIG. 1;

FIG. 10 is a graph of the transfer characteristic of the Glan prism ofthe mastering machine of FIG. 1;

FIG. 11 is a graph of a typical light intensity waveform present in themastering machine of FIG. 1;

FIG. 12, in conjunction with FIG. 11, shows a series of waveforms usefulin explaining the relationship between the intensity of the writing beamand the duty cycle of recording; and

FIG. 13 shows additional waveforms useful in illustrating the operationof the mastering machine of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning first to FIG. 1, the writing apparatus 10 includes a writinghead 12 which is, in the preferred embodiment, a dry microscopeobjective lens 14 mounted upon an air bearing support member 16. A 40Xlens has been found to be satisfactory. A disc 18 is specially preparedand may be constructed according to the teachings of the prior art, inwhich a substrate has coated thereon a very thin film of a metal with areasonably low melting point and a high surface tension.

A crystal oscillator 20 controls the drive elements. The disc 18 isrotated by a first, rotational drive element 22 which is coupled to aspindle 24. A second, translational drive element 26 controls theposition of the writing head 12.

A translating carriage 28, which is driven by the translation driveelement 26 through a lead screw and travelling nut, moves the writinghead 12 in the radial direction relative to the rotating disc 18. Thecarriage 28 is provided with appropriate mirrors and lenses so that theremainder of the optics and electronics necessary to the writing devicemay be permanently mounted.

In the preferred embodiment of the present invention, the beam of apolarized cutting laser 30, which is an argon ion laser, is passedthrough a Pockels cell 32 which is driven by the Pockels cell driver 34.An F.M. modulator 36 receives the video signal that is to be recordedand applies the appropriate control signals to the Pockels cell driver34. As described hereinafter, the video signal is of the typedisplayable on a T.V. monitor. Accordingly, it is a voltage varying withtime signal. The F.M. modulator 36 is of standard design and convertsthe voltage varying with time signal to a frequency modulated signalhaving its informational content in the form of a carrier frequencyhaving frequency changes with time corresponding to said voltagevariations with time. As is known, the Pockels cell 32 responds toapplied signal voltages by rotating the plane of polarization of thelight beam. Since a linear polarizer transmits light only in apredetermined polarization plane, a polarizer, such as a Glan prism 38in the preferred embodiment, is included in the writing beam path toprovide a modulated writing beam 40. The modulated writing beameffectively follows the output of the F.M. modulator 36.

The modulated writing beam 40 emerging from the Pockels cell 32 Glanprism 38 combination is applied to a first mirror 42 which directs thewriting beam 40 to the translating carriage 28. The first mirror 42transmits a portion of the writing beam 40 to a Pockels cell stabilizingcircuit 44 which responds to the average intensity of the writing beamto maintain the energy level of the beam.

A lens 46 is inserted in the path of the writing beam 40 to diverge thesubstantially parallel beam so that it will spread to fill the entranceaperture of the objective lens 14 for optimum resolution. A dichroicmirror 48 is included in the path oriented to substantially transmit allof the writing beam 40 to a second, articulated mirror 50. A mirror,such as has been shown in the patent applications of Elliott may beemployed in the present invention. These applications include Ser. No.299,893 filed Oct. 24, 1972, and entitled "Video Disc Player" nowincluding the issuance of U.S. Pat. No. 3,829,622, and U.S. Pat. No.3,944,727; and Ser. No. 314,082, filed Dec. 11, 1972 and entitled"Improved Video Disc Playback Assembly" now U.S. Pat. No. 3,914,541. Thearticulated mirror 50 then directs the beam through the lens 14 and iscapable of shifting the point of impingement of the beam 40 on thesurface of the disc 18.

A series of holes is formed in the metal coating. One hole is formed foreach cycle of the F.M. modulated signal represented by the modulatedwriting beam 40. Since the modulated writing beam tracks the output ofthe F.M. modulator 36, the holes formed in the coating also track theoutput of the F.M. modulator. Obviously, since the informational contentin the output signal of the F.M. modulator 36 is in the form offrequency changes in time about a carrier frequency, the "hole", "nohole" sequence changes to represent the stored video information by theholes being formed closer or farther apart and the size of the holebecomes larger or smaller as the writing beam 40 changes under thecontrol of the F.M. modulated output signal from the F.M. modulator 36.

The objective lens 14 and the associated air bearing 16 effectively flyon a cushion of air at a substantially fixed distance from the surfaceof the disc 18. That distance is determined by the geometry of thebearing 16, the linear velocity of the disc 18, and the force used toload the head against the disc 18. The fixed spacing is required becausethe focal tolerance of a lens capable of resolving a 1 μm spot is alsoof the order of 1 μm.

A second, relatively low-power laser 52 provides a monitoring beam 54.In the preferred embodiment, the reading laser 52 is a helium-neondevice which enables the reading beam 54 to be distinguished from thewriting beam 40 by wavelength. A polarizing, beam splitter cube 56transmits the reading beam 54 to a mirror 58 that directs the beam 54through a second diverging lens 60 that spreads the reading beam 54 tofill the entrance aperture of the objective lens 14.

A quarterwave plate 62 is placed in the optical path and, in conjunctionwith the plane polarizing beam splitter 56, prevents light reflectedfrom the disc 18 from re-entering the laser 52 and upsetting its mode ofoscillation. The quarterwave plate 62 rotates the plane of polarizationof the beam by 45 degrees on each pass so that the reflected beam isrotated 90 degrees with respect to the polarizing beam splitter 56 andis therefore not passed by it.

A second mirror 64 in the reading beam 54 path directs the beam into thedichroic mirror 48 and is capable of limited adjustment so that thepaths of the writing and reading beams are substantially identical,except that the reading beam "spot" impinges on the disc 18 downstreamfrom the writing beam spot as explained in greater detail below.

A filter 66 that is opaque to the argon ion beam is interposed in thepath of light reflected from the beam splitter 56. The He.Ne readingbeam 54 that is returned from the disc surface is able to pass throughthe filter 66 and through a lens 68 onto a photodetector 70. Thereflected light of the reading beam impinges upon the photodetector 70.The photodetector 70 operates in its standard manner and generates anelectrical current representative of the light impinging thereupon. Inthis case, the photodetector generates the signal represented by the"hole", "no hole" configuration formed in the coating. The "hole", "nohole" configuration is representative of the output of the F.M.modulator 36. The output of the F.M. modulator 36 is a carrier frequencyhaving frequency changes with time representing the video signal to berecorded. The "hole", "no hole" configuration is representative of acarrier frequency having frequency changes with time representing thestored video signal. The output of the photodetector 70 is thus anelectrical signal representing the stored carrier frequency havingfrequency changes with time representing the stored video signal.

The output of the photodetector 70 is applied to a preamplifier 72 whichprovides a signal of sufficient amplitude and signal strength forsubsequent utilization. An F.M. discriminator 74 then provides a videooutput signal which can be utilized in several ways, two of which areshown, as examples only. The discriminator 74 is of standard design andfunction. It takes the frequency modulated signal from the photodetector70 and changes it to a time dependent voltage signal having itsinformational content in the form of a voltage varying with time formatsuitable for display on the T.V. monitor 76.

In a first application, the video output is applied to a T.V. monitor 76and an oscillator 78. As is well known, the T.V. monitor is responsiveto a voltage varying with time signal. The information to be displayedon the T.V. monitor is represented by a voltage change with time. TheT.V. monitor 76 shows the picture fidelity of the recording, and theoscilloscope 78 indicates the signal-to-noise ratio of the record andthe quality of the cutting, whether it is light or heavy. Not shown, anappropriate feedback loop could be provided through the Pockels cellstabilizing circuit 44 to assure an adequate discrimination on the discbetween a "hole" or "black" area and "no hole" or "white" area.

As an alternative utilization, the video output of the discriminator 74is applied to a comparator 80. The other input of the comparator 80 istaken from the video input signal which is directed through a delay line81. A delay that is equal to the accumulated delays of the writingsystem and the time elapsed between the instant of writing of theinformation and the time required for that incremental area of the discto reach the reading point must be imparted to the input video signal.

Ideally, the video output signal of the discriminator 74 should beidentical in all respects to the video input signal, after the properdelay. As previously mentioned, the output from the discriminator 74 isa voltage varying with time signal. The video in signal is also avoltage varying with time signal. Any differences noted represent errorswhich might be caused by imperfections in the disc's surface ormalfunctions of the writing circuits. This application, while essentialif recording digital information, is less critical when otherinformation is recorded.

The output of the comparator circuit 80 can be quantized and counted, sothat an acceptable number of errors can be established for any disc.When the errors counted exceed the standard, the writing operation canbe terminated. If necessary, a new disc can be written. Any disc withexcessive errors can then be reprocessed to serve as a "new" disc for asubsequent recording.

Well-known techniques are available to translate the write head assembly12 in the radial direction with respect to the rotating disc 18. Whilein FIG. 1 the rotational and translational drives 22, 26 are indicatedas independent, the drives are synchronized to enable the writingassembly 12 to translate a predetermined increment for each revolutionof the disc 18, by means of the common crystal oscillator 20.

Turning next to FIG. 2, there is shown in somewhat exaggerated form, theslightly differing optical paths of beam 40 from the writing laser 30and the beam 54 from the reading laser 52. The writing beam 40 coincideswith the optical axis of the microscope objective lens 14. The readingbeam 54, in contrast, makes an angle α with the axis so that it fallssome distance X, equal to α times the focal length of the objective,"downstream" from where the writing beam 40 is "cutting." The resultingdelay between writing and reading allows the molten metal to solidify sothat the recording is read in its final state. If it were read too soonwhile the metal was still molten, it would not provide pertinentinformation for adjusting the recording parameters.

This is best indicated in FIG. 3 where two points in the sameinformation channel are shown as displaced. The point A, which is thepoint of impingement of the writing beam 40, is shown as being on theoptical axis of the objective lens 14. Separated from point A, in thedirection of medium motion, as indicated by the arrow, is the readingpoint B, which is at an angle α from the axis of the microscopeobjective lens 14. A distance between points A and B of two μm hasprovided a satisfactory monitoring of the writing operation.

Turning finally to FIG. 4, there is shown an idealized diagram of aPockels cell stabilizing circuit 44, suitable for use in the apparatusof FIG. 1. As is known, a Pockels cell rotates the plane of polarizationof the applied light as a function of an applied voltage. Therefore, thePockels cell is used to rotate plane polarized light, and the rotatedlight is passed through a plane polarizer, such as a Glan prism. Thelight issuing from the polarizer will be intensity modulated inaccordance with the applied voltage.

Stated another way, the standard operating mode of a Pockels cell 32 andGlan prism 38 combination is for use as a light intensity modulatingmeans. Each cycle from the FM modulator drives the Pockels cell throughits rotational range of ninety degrees. Within this rotational range ofninety degrees, the Glan prism passes the maximum light applied at apoint corresponding to a ninety degree rotation of the light by thePockels cell. The Glan prism passes no light at a point corresponding toa zero rotation of the light by the Pockels cell. The Pockels cell onlyrotates the plane of polarization of the light applied therethrough. TheGlan prism passes maximum light in one plane of polarization and minimumlight in the plane displaced ninety degrees from that plane in which themaximum light passes.

Depending upon the individual Pockels cell, a voltage change ofapproximately 100 volts will cause the cell to rotate the plane ofpolarization through 90 degrees. However, the transfer characteristic ofan individual cell may drift spontaneously, corresponding to a voltagechange of ±50 volts, and accordingly, a feedback loop is desirable tomaintain the cell within a useful, reasonably linear, operating range.

The stabilizing circuit 44 includes a photosensitive silicon diode 82,which is positioned to receive a portion of the writing beam 40transmitted by the mirror 42 of FIG. 1. The silicon diode 82 functionsin much the same fashion as a solar cell and is a source of electricalenergy when illuminated by incident radiation. One terminal of thesilicon diode 82 is connected to common reference potential 84,indicated by the conventional ground symbol and the other terminal isconnected to one input of a differential amplifier 86. The silicon diode82 is shunted by a load 88 which enables a linear response mode.

The other input to the differential amplifier 86 is connected through anappropriate potentiometer 90 to the common reference 84. A source ofpower 92 is coupled to the potentiometer 90, which enables the settingof the differential amplifier 86 to establish the average light leveltransmitted by the Pockels cell 32.

Accordingly, a pair of output terminals of the differential amplifier 86are respectively connected through resistive elements 94, 96 to theinput terminals of the Pockels cell 32 of FIG. 1. It is noted that thePockels cell driver 34 is a.c. coupled to the Pockels cell 32, while thedifferential amplifier 86 is d.c. coupled to the Pockels cell 32.

In operation, the system is energized. The light from the writing beam40 impinging on the silicon diode 82 generates a differential voltage atthe input to the differential amplifier 86. Initially, the potentiometer90 is adjusted to produce light at a predetermined average level ofintensity. Thereafter, if the average level of intensity impinging onthe silicon diode 82 either increases or decreases, a correcting voltagewill be generated in the differential amplifier 86. The correctingvoltage applied to the Pockels cell 32 is of a polarity and magnitudeadequate to restore the average level of intensity to the predeterminedlevel.

Thus, there has been shown an improved video disc recording assembly. Amicroscope objective lens 14 mounted on an air bearing "flies" at apredetermined distance from the surface of a metallized disc 18. Themetallized coating is such that a laser beam 40 can, under suitablemodulation deliver sufficient energy to melt localized areas of thesurface. Under surface tension, the molten metal retracts leaving aclear area of approximately one micron in width.

A second, low-energy laser 52 utilizing substantially the same opticalpath is directed through the same microscope objective lens 17 but isbrought to the surface of the disc at a slight distance "downstream"from the point of writing. The reading beam 54 is returned through anappropriate optical system that excludes the reflected energy of thewriting beam 40 and enables an analysis of the information that has beenwritten on the disc 18.

The playback information can, among other things, control the intensityof the writing beam 40 to assure adequate "recording levels," anddetermine whether or not an unacceptable number of errors have been madein the recording process.

Metal film mastering starts with a fourteen inch diameter glass discwhich is fine ground and optically polished to reduce the pit density towell under one pit per square millimeter. The disc is then washed,thoroughly rinsed in deionized water and spun dry. Then it receives a200 to 300 Angstrom metal coating in a thermal evaporator and is readyfor mastering.

For mastering, the disc is clamped to the spindle of the masteringmachine and the cutting head is moved into position. An air bearingspindle is used. It is driven by a printed circuit type motor and isphase locked to the color subcarrier generated by the video informationsource circuit.

Cutting of the metal coating is accomplished with a microscope objectivelens having 0.75 NA which focuses an argon-ion laser beam to produce asmall spot of light on the disc surface. The spot selectively melts themetal film to encode the disc. The high NA of the lens required toproduce the small (0.8 micron diameter) recording spot, makes the depthof the focus very shallow (plus or minus 0.3 micron) so thatlens-to-metal film distance must be maintained constant to less thanplus or minus 0.3 microns. This is accomplished by mounting the lens ona hydrostatic air bearing which rests on the disc surface. The bearingis loaded with enough force to make it follow disc wobbles as great as25 microns.

The cutting head is driven radially by a lead screw which advances it2.00 microns per turn of the spindle. Since the pitch uniformity of thefinished recording depends on the steady advance of the head, care istaken to lap the lead screw, pre-load the nut which engages it and makethe connection between the nut and the cutting head as stiff aspossible. The cutting process consists of melting holes in metal film tostore the frequency output signal from the FM modulator. The ends of theholes mark the zero crossings of the FM and so all of the FM informationis contained on the disc.

Melting occurs when the power in the light spot exceeds a thresholdcharacteristic of the composition and thickness of the metal film andthe properties of the substrate. The spot power is modulated by aPockels cell driven by the FM encoded video from the FM modulator. Theon-off transitions are kept short to make the location of the hole endsprecise in spite of variations in the melting threshold. Such variationsin the melting threshold can occur due to variations in the thickness ofthe metal coating and/or the use of a different material as theinformation storing layer.

The average power in the spot required to cut a metal layer having athickness between 200 and 300 Angstroms is of the order of 200milliwatts. Since the FM carrier frequency is about 8 MHz, 8×10⁶ holesare cut per second and the energy per hole is 2.5×10⁻⁹ joul. Because themetal solidifies rapidly after melting, it is possible to monitor themastering process through the cutting objective lens by directing a lowpowered (1 milliwatt) reading beam through the objective at an angle toform a reading spot a few microns down stream from the cutting spot. Theread spot is more strongly reflected by the metal than by the exposedglass so the reflected beam can be used to measure the recorded signal,noise, distortion and drop out frequency during cutting. This read whilewrite feature is also used to check disc quality and machineadjustments.

Photoresist mastering is carried out in a manner very similar to metalfilm mastering with a difference that the disc is coated with a thin,uniform layer of positive photoresist instead of metal. The photoresistis exposed directly by the laser spot and development of the photoresistlayer produces pits at the exposed site since positive photoresist isused. At this point, the developed disc is ready for galvanic processingas completely described in U.S. Pat. No. 4,211,617 to C. K. Hunyar,whish is a continuation of application Ser. No. 552,249, filed Feb. 24,1975 and now abandoned.

While photoresist mastering has the advantage of producing a memberwhich is directly usable in the galvanic tooling process, thephotoresist mastering process has a significant drawback in not beingcompatible with the read while write feature. Since the read while writefeature is lost in photoresist mastering, the disc quality is unknownuntil the photoresist is developed and a metal film deposited upon itand the metal covered photoresist master is played back on a player andthe quality monitored on a television monitor.

The ideal information track formed by this mastering process is acircular spiral of constant pitch which makes exactly one turn pertelevision frame. The circularity of the tracks on a master disc isgenerally extremely good compared to the limits imposed by selectivelykeeping the radial acceleration below two gravities.

Time displacement error is largely dependent on the precision of themastering spindle except for errors introduced by eccentricity andunroundness made in the replication process. The spindle in themastering process is phase locked to the color subcarrier so that on theaverage the mastering spindle makes exactly one turn per televisionframe. Testing has shown that this procedure results in a spindle timeerror under 14 nanoseconds when sampled at 30 Hz.

Prior to giving the detailed mode of operation of the mastering machine,it would do well to establish a number of terms which have a specialmeaning in the description contained hereinafter. The laser intensitygenerated by the writing laser source as it impinges upon the mastervideo disc is employed to interact with the information bearing portionof the video disc to form indicia representing the carrier frequency andthe frequency variations in time from the carrier frequency.

The threshold power level required of the laser beam at the point ofimpact with the information bearing layer of the video disc differsdepending upon the material from which the information bearing layer ismade. In the two examples given hereinabove, bismuth and photoresist,the threshold power level required to form indicia differs significantlyand represents a good example for illustrating the term threshold power.Obviously, the threshold power of other materials would also differ fromeach of the examples explained.

The indicia formed in a bismuth coated video disc master are alternateregions of light reflectivity and light non-reflectivity. The areas oflight non-reflectivity are caused by the melting of the bismuth followedby the retracting of the bismuth before cooling to expose an underlyingportion of the glass substrate. Light impinging upon the metal layer ishighly reflected while light impinging upon the exposed portion of theglass substrate is absorbed and hence light non-reflectivity isachieved.

The threshold power is that power from the laser beam required toachieve melting and retracting of the metal layer in the presence of alaser beam of increasing light intensity. The threshold power level isalso represented as that intensity of a decreasing light intensitysignal when the metal layer ceases to melt and retract from the regionhaving incident light impinging thereupon. More specifically, when thepower in the impinging light beam exceeds the threshold powerrequirements of the recording material, a hole is formed in therecording material. When the light power intensity in the impinginglight beam is below the threshold power level of the recording material,no hole is formed in the recording medium. The forming of a hole and thenon-forming of a hole by the impinging light beam is the principalmanner in which the light beam impinging upon a bismuth coated masterinteracts with the bismuth layer to form indicia on the recordingsurface. The indicia represents a carrier frequency having frequencychanges in time varying about the carrier frequency.

A video disc master having a thin layer of photoresist formed thereoverhas its own threshold power level. The mechanism whereby a light beamexposes a photoresist layer is pursuant to a photon theory requiring asufficient number of photons in the impinging light beam to expose aportion of the photoresist. When the positive going modulated light beamcontains sufficient photons above this threshold power level, thephotoresist in that area is exposed so that subsequent developmentremoves the exposed photoresist. When the photon level in a decreasinglight intensity modulated light beam falls below the normal thresholdpower level of the photoresist, the photoresist ceases to be exposed tothe extent that subsequent development does not remove the photoresistilluminated by an impinging light beam having photons below thethreshold power level.

The impinging light beam from the modulated laser source interacts withthe information bearing layer to fully expose or under expose thephotoresist layer illuminated by the impinging light beam. This is aninteraction of the photons in the impinging light beam with theinformation bearing member to form indicia of the carrier frequencyhaving frequency changes in time varying about the carrier frequency.The indicia storing the carrier frequency and frequency change in timeare more fully appreciated after the development step whereby thoseportions of fully exposed photoresist material are effectively removedleaving the under exposed portions on the video disc member.

Referring to line A of FIG. 5, there is shown an idealized or simplifiedvideo waveform that is typically supplied as a video signal from a videotape recorder or television camera. This video signal is then applied toan FM modulator circuit 36. Two output signals are shown on lines B andC, and each is an FM modulated output signal and each carries the samefrequency information. The waveform on line B shows the output normallygenerated by a multi-vibrator type FM modulator 36. The waveform shownon line C shows the output generated by an FM modulator 36 having atriangular shaped output waveform. Both waveforms contain the samefrequency information. The triangular shaped waveform gives enhancedresults when used in driving a Pockels cell 32 for light modulation of aconstant intensity light beam applied through the Pockels cell.

A device for producing a suitable triangular waveshape signal ismanufactured by Wavetek, Inc. of San Diego, CA., Model 164.

The frequencies contained in each waveform B and C are at all timesidentical and each represents the voltage level of the video waveformshown in line A. By inspection, it can be seen that the lower amplituderegion of the video waveform generally indicated by the numeral 200corresponds to the low carrier frequencies and higher amplitude regionsof the video waveform as generally indicated at 202 corresponds to thehigher frequency shown in lines B and C. It is the custom and practiceof the television industry to utilize a one volt peak to peak voltagesignal having voltage variations in time as the video signal generatedby a television camera. These signal characteristics are the samerequired to drive a television monitor 76. The advantage of using atriangular shaped waveform for driving a Pockels cell 32 is to match thePockels cell's transfer characteristic with a selected waveform of themodulating signal to achieve a sinusoidal modulation of the light beampassing through the Pockels cell and to the Glan prism 38. Thetriangular waveform shown in line C is a linear voltage change withtime. The linear voltage change versus time of the triangular drivingwaveform when multiplied by a sinusoidal voltage change versus lighttransfer function of the Pockels cell 32 gives a sinusoidally varyinglight intensity output from the Glan prism.

The waveform shown on line D illustrates the sinusoidal waveform whichcorresponds to the light intensity output from the Glan prism 38 whenthe Pockels cell 32 is driven by the triangular waveform shown on lineC.

Referring specifically to the bottommost point at 204 and the topmostpoint at 206 of the waveform shown on line D, the point exactly equallydistant from each is identified as the half power point. Anunderstanding of the utilization of this half power point feature isrequired for high quality mastering operations.

The peak to peak voltage of the triangular waveform is represented by afirst maximum voltage level V₂ shown at bint 208 on line C and by asecond minimum voltage level V₁ on line 210. The voltage differentialbetween points 208 and 210 is the driving voltage for the Pockels cell32. This voltage differential is adjusted to equal that voltage requiredby the Pockels cell 32 to give a ninety degree rotation of the outputpolarization of the light passing through the Pockels cell 32. The biason the Pockels cell is maintained such that voltage levels V₁ and V₂always correspond to the zero degree rotation and the ninety degreerotation respectively of the light beam passing through the Pockels cell32. The forty-five degree rotation of a light beam is half way betweenthe two extremes of a triangle waveform. That half way voltage is alwaysthe same for the Pockels cell 32. But the half way voltage with respectto zero volts may drift due to thermal instabilities causing the halfpower voltage point to drift also. The correct biasing of the halfwayvoltage is completely described hereinafter with reference to FIGS. 9,10 and 11.

While the waveform shown on line C of FIG. 5 shows the triangular waveshape generated by the FM modulator 36, it also represents the waveshape of the signal generated by the Pockels cell driver 34. The outputfrom the FM modulator is typically in a smaller voltage range, typicallyunder 10 volts while the output from the Pockels cell driver typicallyswings 100 volts in order to provide suitable driving voltage to thePockels cell to drive it from its zero rotational state to its ninetydegree rotational state. In discussing the voltage levels V₁ and V₂ andthe lines 210 and 208, respectively, representing such voltage points,reference is made to line C of FIG. 5 because the output from thePockels cell driver has the identical shape while differing in theamplitude of the waveform. This was done for convenience and theelimination of a substantially identical waveform different only inamplitude.

Referring to FIG. 6, there is shown a cross sectional, schematic view ofa video disc formed according to the mastering process of the inventiondescribed herein. A substrate member is shown at 300 having a planarupper surface indicated at 302. An information bearing layer 304 isformed to top the upper surface 302 of the substrate 300. Theinformation bearing layer 304 is of uniform thickness over the entiresurface 302 of the substrate 300. The information layer 304 itself has aplanar shaped upper surface 306.

FIG. 6 is shown positioned beneath Line C of FIG. 5 showing theintensity of the light beam passing from the Pockels cell 32 Glan prism38 combination in the improved embodiment which utilizes a voltagecontrolled oscillator in the F.M. modulator 36 for generating atriangular shaped output waveform as the driving waveform shape to thePockels cell. As previously described, the threshold power level of theinformation bearing layer is defined as that power required to formindicia in the information bearing layer in response to the impinginglight beam. For a metal surface, the thermal threshold point is thatpower required to melt the metal layer and have the metal layer retractfrom the heated region of impingement. For a photoresist layer, thethreshold power level is that power level required to supply sufficientphotons to completely expose the photoresist information bearing layer.In the case of the metal layer, the heated metal retracts from theimpinging area to expose the substrate 300 disposed thereunder. In thecase of the photoresist material, the photon power is sufficient tofully expose the total thickness of the photoresist layer 324 completelydown to the upper surface 322 of the substrate 320 as described withreference to FIG. 7.

It has been previously discussed how the half power point of the Pockelscell 32 Glan prism 38 combination is located at a point halfway betweena first operating point at which maximum transmission from a fixedintensity beam passes through the Glan prism and a second operatingpoint at which minimum transmission from a fixed intensity beam passesthrough the Glan prism. The half power point is the point at which thelight passing through the Pockels cell has been rotated forty-fivedegrees from the point of zero power transmission.

In operation, the output power from the laser 30 is adjusted such thatthe half power point of the Pockels cell 32 Glan prism 38 combinationprovides sufficient energy to equal the threshold power level of theinformation bearing member employed, such as the member 304. Thematching of the half power point of the Pockels cell-Glan prismcombination ensures highest recording fidelity of the video frequencysignal to be recorded and ensures minimum intermodulation distortion ofthe signal played back from the video disc recording member.

This matching of the power levels is illustrated with reference to lineD of FIG. 5 and FIG. 6 and by the construction lines drawn verticallybetween the half power point represented by the line 214 shown on line Dof FIG. 5 and the apertures shown generally at 310 in FIG. 6. The lengthof an aperture 310 is coextensive with the time that the transmittedintensity of the modulated light beam exceeds the half power point lineshown with reference to line D of FIG. 5.

In this embodiment, the half power point line 214 also represents thezero crossing of the triangular wave shape shown on line C of FIG. 5.The zero crossing points are represented by lines 216 and 216' in FIGS.5B and 5C, and the importance of regulating the half power point isexplained in greater detail with reference to FIGS. 11 and 12.

FIG. 7 shows an information storage member including a substrate 320having a planar upper surface 322. A thin layer of photoresist 324 ofuniform thickness is formed over the planar upper surface 322 of thesubstrate 320. The thin photoresist layer 324 is also formed with aplanar upper surface 326. The photoresist layer 324 is a lightresponsive layer just as the metal bismuth layer 304 is a lightresponsive layer. Both the thin opaque metallized coating 304 and thephotoresist layer 324 function to retain indicia representative of thevideo input signal. In the case of the metal layer 304, apertures 310are formed in the metallized layer to form successive light reflectiveand light non-reflective regions in the information storage member.

Referring to FIG. 8 showing the photoresist coated information storagemember, regions 330 are formed in substantially the same manner asregions 310 were formed with reference to the structure shown in FIG. 6.Rather than apertures 310 being formed as shown with reference to FIG.6, exposed regions 330 are formed corresponding to the apertures 310.The exposed photoresist material is represented in FIG. 7 by crosshatching of the regions in the photoresist information storage layer324. Subsequent development of the exposed photoresist material removessuch exposed photoresist material leaving apertures comparable to theapertures 310 shown with reference to FIG. 6.

In operation, when using the photoresist coated substrate video discmember, the output power of the writing laser is adjusted such that thepower of the modulated laser beam 40 passing through the Pockels cell 32Glan prism 38 combination at the half power point of the Glan prismequals the photon threshold power required to completely expose thephotoresist illuminated by the impinging light beam. Just as with thebismuth coated master video disc system, this ensures highest fidelityrecording and minimum intermodulation distortion during the playback ofthe recorded video signal.

In referring to both FIGS. 6 and 8, that portion of the light beampassing through the Glan prism 38 above the half power point of the Glanprism as represented by that portion of the waveform shown on line D ofFIG. 5 which is above the line 214, causes an irreversible change in thecharacteristics of the light sensitive surface 304 in the case of thebismuth coated video disc shown in FIG. 6 and the photoresist coating324 shown with reference to the photoresist coated video disc shown inFIG. 7. In the case of the bismuth coated video disc member 300, theirreversible changes take the form of successively formed apertures 310in the opaque metallized coating 304. In the case of the photoresistcoated substrate 320, the irreversible alteration of the characteristicof the photoresist layer 324 occurs in the form of successive fullyexposed regions 332.

While bismuth is listed as the preferred metal layer, other metals canbe used such as tellurium, inconel and nickel.

Referring to FIG. 9, there is shown the transfer characteristic of thePockels cell 32 and Glan prism 38 combination as a sinusoidal voltagechange versus rotation in degrees of the light passing through thePockels cell 32 versus linear voltage change of input drive to thePockels cell. The ninety degree rotation point is shown at point 340 andequals the maximum light transmission through the Glan prism. The zerodegree rotation point is shown at points 342 and equals the zero orminimum light transmission through the Glan prism. The zero lighttransmission point 342 corresponds to the voltage level V₁ representedby the line 210 in line C of FIG. 5. The ninety degree rotation pointcorresponds with the voltage level V₂ represented by the line 208 online C of FIG. 5. The point half way between these two voltagesrepresented by the line 216' is equal to V₂ minus V₁ /2 and correspondsto a forty-five degree rotation of the light beam passing through thePockels cell.

As is well known, the power through the Pockels cell is substantiallyunchanged. The only characteristics being changed in the Pockels cell 32is the degree of rotation of the light passing therethrough. In normalpractice, a Pockels cell and Glan prism 38 are used together to achievelight modulation. In order to do this, the principal axis of the Pockelscell and the Glan prism are put into alignment such that a light beampolarized at ninety degrees rotation passes substantially undiminishedthrough the Glan prism. When the same highly polarized light is rotatedby the Pockels cell for ninety degrees rotation back to the zero degreerotation, the light beam does not pass through the Glan prism. In actualpractice, the full transmission state and zero transmission state is notreached at high frequencies of operations.

Referring to FIG. 10, there is shown the transfer characteristic of aGlan prism. At point 350, maximum transmission through the Glan prism 38is achieved with a ninety degree rotation of the incoming light beam. Atpoint 352, minimum or zero light transmission through the Glan prism isachieved at zero rotation of the incoming light beam. Half of theintensity of the impinging light beam is passed through the Glan prismas indicated at points 354 which corresponds to forty-five degreesrotation of the light entering the Glan prism. Obviously, the absolutepower of the light passing through the Glan prism at the forty-fivedegree rotation can be adjusted by adjusting the light output intensityof the light source. In this embodiment, the light source is the writinglaser 30.

In the preferred embodiment, the power output from the writing laser 30is adjusted such that the intensity of the light passing through theGlan prism 38 at its half power point coincides with the threshold powerlevel of the recording medium. Since more power is required to melt abismuth layer than is required to fully expose a photoresist layer, theabsolute intensity of a writing beam used in writing on a bismuth masterdisc is greater than the intensity of a writing laser used to interactwith a photoresist covered master video disc.

Referring collectively to FIGS. 11 and 12, there is shown a series ofwaveforms useful in explaining the relationship between length of a holecut in a master video disc by the writing laser 30 and the length ofuncut land area between successively formed holes. This relationship hasbeen referred to hereinbefore as a relationship formed by the value ofthe peak cutting power, the average cutting power and focus of the spoton the metal layer. Collectively, these terms have evolved into a singleterm known as duty cycle which term represents all three suchcharacteristics.

As previously described, the energy required to interact with theinformation bearing layer on video disc substrate is that energynecessary to cause irreversible changes in the material selected forplacement on the master video disc member. In the case of a bismuthcoated master, the energy required is that needed to selectively removethe portion of the bismuth coated layer in those locations when theenergy is above the threshold energy level of the bismuth layer. If thisenergy contained in the focused spot of light is not focused properlyupon the bismuth layer, then the energy cannot be used for its intendedfunction and it will be dissipated without effecting its intendedfunction. If some cutting occurs, distortions are introduced into themastering process. If the peak cutting power greatly exceeds thethreshold power level of the recording medium, destructive removal ofmaterial occurs and provides a surface containing distortion productscaused by this destructive removal. The average cutting power is thatpower at a point midway between a first higher cutting power and asecond lower cutting power. As just described, the average cutting poweris preferably fixed to equal the threshold power level of the recordingmedium. In this sense, the intensity of the light beam above the averagecutting power interacts with the information bearing layer to formindicia of the signal to be recorded. The intensity of the light beambelow the average cutting power fails to heat a bismuth coated master toa point needed in the hole forming process or fails to fully expose aportion of a photoresist coated master.

Referring briefly to lines B and C of FIG. 5, the adjustment of theaverage cutting power to coincide with the line 216 shown in line B andwith the line 216' shown with reference to the line C of FIG. 5, resultsin a duty cycle where the length of a hole equals the length of the"land" area positioned successively thereafter. This is known as a 50%or fifty-fifty duty cycle. A fifty-fifty duty cycle is the preferableduty cycle in a recording procedure but commercially acceptable playbacksignals can be achieved in the range from sixty-forty to forty-sixty.This means that either the hole or the intervening land member becomeslarger while the other member becomes smaller.

Referring to FIG. 11, there is shown a waveform represented by a line360 corresponding to two cycles of the light intensity transmittedthrough the Pockels cell-Glan prism combination and represented morespecifically on line D of FIG. 5. The threshold power level of therecording medium is represented by a line 362. The threshold power levelof the reading medium is caused to be equal to the half power point ofthe light intensity transmitted by the Pockels cell-Glan prismcombination by adjusting the absolute intensity of the writing laser 30.

When the threshold level is properly adjusted at the half power point,an indicia is formed on the information surface layer of the mastervideo disc beginning at point 364 and continuing for the time until theintensity falls to a point 366. Dash lines shown at 364' and 366' aredrawn to line A of FIG. 12 showing an indicia represented by the ellipse368, which has been formed for the period of time when the lightintensity continues to rise pass the point 364 to a maximum at 370 andthen falls to a point 366. The light intensity below point 366 falls toa minimum at 372 and continues to rise towards a new maximum at 374. Ata certain point between the lower intensity level 372 and the upperintensity level 374, the light intensity equals the threshold powerlevel of the recording medium at 376. Beginning at point 376, the energyin the light beam begins to form an indicia represented by the ellipse378 shown on line A of FIG. 11. A dotted line 376' shows the start offormation of indicia 378 at the point when the light intensity exceedsthe threshold level 362. The indicia 378 continues to be formed whilethe light intensity reaches a maximum at 374 and begins to fall to a newminimum at 375. However, at the intersection of line 360 with thethreshold power level at 362 the light intensity falls below thethreshold power level and the indicia is no longer formed. In thepreferred embodiment, the length of the indicia represented by a line384 equals the length of the land region shown generally at 386 asrepresented by the length of the line 388. Accordingly, the matching ofthe half power point light intensity output from the Pockels cell 32Glan prism 38 combination with the threshold power level of therecording surface results in a fifty-fifty duty cycle wherein the lengthof the indicia 368 equals the length of the next succeeding land region386. Points 364, 366, 376 and 382 shown on the line 360 represent thezero crossing of the original frequency modulated video signal. Hence,it can be seen how the indicia 368 and 386 represent the frequencymodulated video signal. This representation in the preferred embodimentrepresents a fifty-fifty duty cycle and is achieved by adjusting thehalf power level of the beam 40 exiting from the Pockels cell-Glan prismcombination to equal the threshold power level of the recording medium.The waveform shown with reference to FIG. 11, including the variablelight intensity represented by the line 360, represents a preferred modeof operation to achieve 50/50 duty cycle independent of the recordingmedium employed on the master video disc member. The absoluteintensities at the various points change according to the absoluteintensities required for the modulated light beam to interact with therecording surface, but the relative wave shapes and their relativelocations remain the same. More specifically, the absolute intensity ofthe threshold power level for bismuth is different than the absoluteintensity of the threshold power level for photoresist, but therelationship with the intensity line 360 is the same.

Referring to the combination of FIG. 11 and line B of FIG. 12, therewill be described the results of failing to match the half power pointoutput of the Pockels cell-Glan Prism combination with the thresholdpower level of the recording medium. Referring to FIG. 11, a second dashline 380 represents the relationship between the actual threshold powerlevel of the recording medium being used with the light intensity outputfrom the Pockels cell 32 Glan Prism 38 combination. The threshold powerlevel line 380 intersects the intensity line 360 at a plurality oflocations 390, 392, 394 and 396. A line 390' represents the intersectionof the light intensity line 360 with the threshold power level 380 andsignals the start of the formation of an indicia 398 shown on line B ofFIG. 12. The indicia 398 is formed during the time that the lightintensity is above the threshold power level. The length of the indicia398 is represented by the time required for the light intensity to moveto its maximum at 370 and fall to the threshold point 392 as is shown bya line 399. The length of a land area indicated generally at 400 has alength represented by a line 402. The length of a line 402 is determinedby the time required for the light intensity to move from thresholdpoint 392 to the next threshold point 394. During this time, theintensity of the light beam is sufficiently low as to cause nointeraction with the recording medium. A second indicia is shown at 406and its length corresponds with the point at which the intensity of thewaveform represented by the line 360 exceeds the threshold power levelat point 394. The length of the indicia 406 is shown by a line 408 andis determined by the time required for the light intensity to rise to amaximum at 374 and fall to the threshold level at point 396.

Various lines are shown indicating the beginning and ending of theindicia and intravening land areas by employing the number raise to theprime used to identify the intersection of the light intensity line 360with the threshold power level lines 362 and 380.

The successively positioned indicia 398 and land region 400 represent asingle cycle of the recorded frequency modulated video signal. Theindicia 398 represents approximately 65 percent of the sum of the lengthof the line 399 and the line 402. This represents a duty cycle of 65/35.Sixty-five percent of the available space is an indicia whilethirty-five percent of the available space is land area. Typically, theindicia in the final format is a light scattering member such as a bumpor hole, and the land area is planar surface covered with a highlyreflective material.

The frequency modulated video information represented by thesequentially positioned light non-reflective member 368 and lightreflective member 386 shown in line A (FIG. 12) represents the preferredduty cycle of 50/50. When the photoresist mastering procedure isemployed, the reflectivity of the upper surface of the photoresist layeris not significantly altered by the impingement of the writing beam suchas to be able to detect a difference between light beams reflected bythe exposed and non exposed portions of the photoresist member. It isbecause of this effect that a read after write procedure, using aphotoresist coated master video disc is not possible.

Referring to line C of FIG. 12, there is shown a representation of therecovered video signal represented by the sequence of indicia 368 andland area 386 shown on line A. The waveform shown in line C is anundistorted sine wave 410 and contains the same undistorted frequencymodulated information as represented by the light intensity waveformrepresented by the line 360 shown in FIG. 11. The sine wave shown inline C of FIG. 12 has a center line represented by a line 412 whichintersects the sine wave 410 in the same points of intersection as theline 362 intersects the intensity line 360 shown in FIG. 11.

Referring to line D of FIG. 12, there is shown a recovered frequencymodulated video signal having bad second harmonic distortion. Thefundamental frequency of the waveform represented by a line 414 shown inline D is the same as that contained in the waveform shown one line C.However, the information shown in line D contains bad second harmonicdistortion. When used in a system in which bad second harmonicdistortion is not a disabling problem, the attention to a 50/50 dutycycle situation explained hereinabove need not be strictly followed.However, when it is necessary to have a substantially undistorted outputsignal recovered from the video disc surface, it is necessary to followthe procedure described hereinabove.

Referring to FIG. 13, there is shown the relationship between theintensity of the reading spot in the reading beam as it impinges uponsuccessively positioned light reflective and light non-reflectiveregions formed during a preferred form of the mastering process. In apreferred embodiment, a metal is used for this purpose and the preferredmetal as disclosed is bismuth.

Line A of FIG. 13 shows a plurality of indicia formed in the surface ofa video disc master. In the preferred embodiment the holes formed in abismuth layer 420 are shown at 422, 424 and 426. The interveningportions of the layer 420 which are unaffected by the formation of theholes 422, 424 and 426 are sometimes called "land" areas and areindicated generally at 428 and 430. The land areas are highlyreflective. The formation of the holes 422, 424 and 426 expose theunderlying glass substrate which is essentially light absorbing andhence the glass substrate is a light non-reflective region. The waveformshown at 432 represents the light intensity waveform of the spot in theread beam as the spot passes over a light non-reflective region. Thisindicates the spacial relationship between the spot as it moves over alight non-reflective region.

Referring to line B of FIG. 13, there is shown a waveform represented bya line 434 indicating the intensity waveform of the reflected light as aspot having the intensity relationship shown in FIG. A passes over asuccessively positioned light reflective and light non-reflectiveregion. A solid line portion 436 of the line 434 shows the intensitywaveform of the reflected light as the spot passes over the lightnon-reflective region 424. The intensity of the reflected light shows aminimum at point 438 which corresponds with the center of thenon-reflective region 424. The center of the non-reflective portion 424is shown on a line 440 at a point 442. The intensity waveform of thereflected light is a maximum, as shown at 444, when corresponding to acenter point 446 of the land area 428 positioned between successivenon-reflective regions 422 and 424 respectively. The center point 446 isshown on a line 448 representing the center line of the informationtrack. The dotted portion of the line 434 represents the past history ofthe intensity waveform of the reflected light when the light passed overthe non-reflective region 422. A dotted portion 452 of the waveform 434shows the expected intensity of the reflected light beam when thereading spot passes over the non-reflective region 426.

Referring to line C of FIG. 13, there is shown the recovered electricalrepresentation of the light intensity signal shown on line B. Theelectrical representation is shown as a line 454 and is generated in thephotodetector 70 shown in FIG. 1.

A special advantage of the read while write capability of the masteringprocedure herein described includes the use of the instantaneousmonitoring of the information just written as a means for controllingthe duty cycle of the reflective and non-reflective regions. Bydisplaying the recovered frequency modulated video signal on atelevision monitor during the writing procedure, the duty cycle can bemonitored. Any indication of the distortion visible on the monitorindicates that a change in duty cycle has occurred. Means are providedfor adjusting the duty cycle of the written information to eliminate thedistortion by adjusting the duty cycle to its 50/50 preferred operatingpoint. A change in duty cycle is typically corrected by adjusting theabsolute intensity of the light beam generated in the laser 30 in asystem having either an average intensity biasing servo or a secondharmonic biasing servo and in conjunction with circuitry for adjustingthe half power point output of the Pockets Cell-Glan Prism combinationto equal the threshold power level of the recording medium. The termhalf power point and average intensity are interchanged in the portionsof the specifications and claims which concern the use of the triangularshaped wave form generated by the FM modulator. The modulated light beam40 exiting from the Glan Prism 38 is of sinusoidal shape. In thissituation the half power point equals the average intensity and thiswould be the case for any symmetrical wave form. A frequency modulatedoutput from an FM modulator has been found to act as such a symmetricalwave form.

While the invention has been particularly shown and described withreference to a preferred embodiment and alterations thereto, it would beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention.

What is claimed is:
 1. An apparatus for recording an electricalinformation signal by directing a concentrated high intensity laserlight write beam at a recording surface, including means responsive tothe electrical signal for controlling the intensity of the laser beamimpinging on the recording surface, said apparatus comprising:aninformation storage member having a substrate and a thin,light-sensitive surface layer overlying said substrate; means for movingsaid storage member in a prescribed fashion; a laser light source forproviding a write beam of light having sufficient intensity to interactwith said light-sensitive layer and produce indicia representative ofthe information signal; said thin surface layer having a substantiallyuniform threshold power level; said threshold power level being of thetype wherein said layer reacts to said laser write beam having anintensity greater than said threshold power level and said layer doesnot react to said laser write beam having an intensity less than saidthreshold power level; an optical modulator positioned in said laserwrite beam intermediate said thin surface layer and responsive to theelectrical signal for modulating the intensity of the laser write beamto include a predetermined first intensity at which said beam forms anindicia in said thin surface layer and a predetermined second intensityat which said beam does not form an indicia in said thin surface layer;and feedback apparatus for stabilizing the operating level of saidoptical modulator to issue said write beam at a predetermined averagepower level equal to the threshold power level of said layer.
 2. Anapparatus as claimed in claim 1, wherein said feedback apparatus furtherincludes:light-sensing means for sensing at least a portion of saidlaser write beam issuing from said optical modulator and responsivethereto for producing and applying to said optical modulator acorresponding bias signal to stabilize the operating level of saidoptical modulator.
 3. An apparatus as claimed in claim 1, and furtherincluding:said intensity of said laser light beam being adjustablerelative to said threshold power level of said light-sensitive surface.4. The apparatus as claimed in claim 1, wherein the surface layer is ametallic layer.
 5. The apparatus as claimed in claim 1, wherein thesurface layer is a photoresist layer.
 6. The apparatus as claimed inclaim 1, wherein said feedback apparatus includes:first bias meansresponsive to the average intensity level of said intensity of saidmodulated light transmitted through said optical modulator for adjustingsaid average intensity of said modulated light beam at a fixed firstlevel; and second bias means responsive to a change in average intensitylevel established in said modulated light transmitted through saidoptical modulator for restoring the changed intensity level to saidfixed first level.
 7. Apparatus for storing an electrical signal upon aninformation storage member, comprising:first means for providing aninformation signal to be recorded; said signal having its informationalcontent in the form of a variable amplitude, cyclical signal alternatingbetween a first higher amplitude and a second lower amplitude; aninformation storage member including a substrate having a first surfaceand a light responsive coating covering said first surface for retainingindicia representative of said information signal; said coating having athreshold power level above which said indicia are formed; means forimparting uniform motion to said storage member; a laser light sourcefor providing a write light beam, and said write beam being ofsufficient intensity for interacting with said coating while saidcoating is in motion and said coating is positioned upon said movinginformation storage member, and said light beam being of sufficientintensity for altering said coating to retain indicia representative ofsaid information signal; said intensity of said light beam having apredetermined value relative to said threshold power level of saidcoating; optical means for defining an optical path between said lightsource and said storage member including said coating, and said opticalmeans being further employed for imaging said light beam to a spot uponsaid coating; light intensity modulating means positioned in saidoptical path between said light source and said coating, and said lightintensity modulating means operating over a range between a maximumlight transmitting state and a minimum light transmitting state forintensity modulating said light beam with said information to be stored;said light intensity modulating means being responsive to saidinformation signal and changing between its maximum light transmittingstate and its minimum light transmitting state during each cycle of saidinformation signal for modulating said light beam with the informationsignal to be stored; said light intensity modulating means has anintermediate light transmitting state at which the instantaneous powerin said modulated light beam equals one half that of said modulatedlight beam transmitted at said maximum state and said intermediate lighttransmitting state also equals the threshold power level of saidcoating; and said light passing through said light intensity modulatingmeans and imaged upon said coating by said optical means begins to formindicia in said coating representative of said information signal to bestored when said intermediate light transmitting state is exceeded. 8.The apparatus as claimed in claim 7, and further including:stabilizingmeans responsive to the average intensity of said modulated lighttransmitted through said light intensity modulating means for generatinga bias control signal for application to said light intensity modulatingmeans, said bias control signal being employed for maintaining saidaverage intensity of said modulated light beam at a prescribed level. 9.The apparatus as claimed in claim 8, wherein said stabilizing meansfurther includes:first bias means responsive to the average intensitylevel of said intensity of said modulated light transmitted through saidlight intensity modulating means for adjusting said average intensity ofsaid modulated light beam at a fixed first level; said average intensitybeing set equal to said intensity present at said intermediate lighttransmitting state; and second bias means responsive to a change inaverage intensity level established in said modulated light transmittedthrough said light intensity modulating means for restoring said changedintensity level to said fixed first level.
 10. The apparatus as claimedin claim 7 and further comprising:said light beam issuing from saidlight intensity modulating means during the period corresponding to theperiod said light beam exceeds said half power level being of sufficientintensity for interacting with said light responsive coating while saidsurface is in motion to impart a permanent and substantially uniformphysical change during said period, representative of said informationsignal; said light beam issuing from said light intensity modulatingmeans during the period corresponding to the time said beam is at apower level less than said half power level having an intensity lessthan the intensity required for altering said light responsive coating;said maximum light transmitting state corresponding to said firstvoltage level of said information signal, and said minimum lighttransmitting state corresponding with said second voltage level of saidinformation signal;said light beam passing through said light intensitymodulating means and focused upon said light responsive coating by saidoptical means having sufficient intensity to form a first permanent andsubstantially uniform physical change in said surface for a firstportion of each cycle of said information signal during the time thatsaid information signal is at said first voltage level; and said lightbeam passing through said light intensity modulating means and imagedupon said light responsive coating by said optical means falling belowsaid intensity required to form a physical change in said surface forthe remaining portion of each cycle of said information signal duringthe time that said information signal is at said second voltage level.11. The apparatus as claimed in claim 7, and further comprising:saidintensity of said light beam being adjustable relative to said thresholdpower level of said coating.
 12. A method for recording an informationsignal on a rotatable recording disc, comprising the steps of:providingan electrical signal to be recorded, and said signal having itsinformational content in the form of a variable amplitude, cyclicalsignal alternating between a first higher amplitude and a second loweramplitude; providing a rotatable recording disc having a substrate and athin, light-sensitive coating overlying the substrate; rotating therecording disc in a prescribed fashion; providing a beam of light havingsufficient intensity to interact with said light-sensitive coating andproduce indicia representative of the information signal; directing thebeam of light along an optical path to impinge on said light-sensitivecoating; controllably moving the point of impingement of the beam oflight on said recording disc radially in a prescribed fashion, such thatthe beam of light impinges on the rotating disc in a succession ofsubstantially circular and concentric recording tracks; modulating theintensity of the beam of light in accordance with the information signalto be recorded, the intensity varying between a prescribed maximumintensity and a prescribed minimum intensity during each cycle of thesignal, the prescribed maximum intensity corresponds to the first higheramplitude portion of the information signal and is greater than apredetermined recording threshold of said light-sensitive coating, andthe prescribed minimum intensity corresponds to the second loweramplitide portion of the informative signal and is less than thepredetermined recording threshold; and stabilizing the average intensityof the intensity-modulated beam of light at a prescribed levelcorresponding both to one-half the prescribed maximum intensity and tothe predetermined recording threshold of said light-sensitive coating,the indicia being arranged in a succession of substantially circular andconcentric recording tracks.
 13. A method as defined in claim 12,wherein said step of stabilizing includes the steps of:monitoring theintensity-modulated beam of light and producing an average intensitysignal representative of the average intensity of the beam; comparingthe average intensity signal with a prescribed selectable level, toproduce a difference signal representative of the differencetherebetween; and using the difference signal in said step ofmodulating, to controllably adjust the average intensity of theintensity-modulated beam of light accordingly.
 14. A method as definedin claim 13, wherein:the beam of light provided in said second step ofproviding is linearly polarized; said step of modulating includes a stepof rotating the polarization plane of the beam of light through an angleof 90 degrees during each cycle of the information signal to berecorded; and said step of stabilizing stabilizes said light intensitymodulating means such that the midpoint in the 90 degree range ofrotation of the polarization plane of the beam of light results in anintensity corresponding to the predetermined recording threshold of saidlight-sensitive coating.
 15. A method as defined in claim 12, whereineach of the successive indicia formed in said light-sensitive coatinghas a length substantially equal to that of the adjacent space betweensuccessive indicia.
 16. A method for recording information on aninformation storage member using a laser beam, comprising the stepsof:providing an electrical signal to be recorded, and said signal havingits informational content in the form of a variable amplitude, cyclicalsignal alternating between a first higher amplitude and a second loweramplitude; using said electrical signal as a control signal forcontrolling the application of a light beam upon a light sensitivesurface of an information storage member; adjusting the averageintensity of said light beam to equal the threshold power level of saidlight sensitive surface; moving the information storage member at aconstant rate while focusing said stationary light beam to a spot uponthe light sensitive surface of said information storage member; usingsaid focused light spot to irreversibly alter the characteristics ofsaid light sensitive surface of said information storage member as saidmember moves at a constant rate and under the control of said higheramplitude portion of said information signal; and blocking thetransmission of said focused light beam to said light sensitive surfaceof said information storage member as said member moves at a constantrate and under the control of said lower amplitude portion of saidinformation signal.
 17. The method as claimed in claim 16, including thesteps of:generating a fixed intensity beam of light; using saidinformation signal as a control signal for varying the amount of saidfixed intensity light beam impinging upon a light sensitive surface ofan information storage member; selecting said fixed intensity of saidlight beam in said adjusting step to provide an average intensity of themodulated light beam to equal the threshold power level of said lightsensitive surface; and stabilizing the average intensity of said beam oflight to equal the threshold power level of said light sensitivesurface.